A prior study contrasted the Myalgic Encephalomyelitis International Consensus Criteria with the Fukuda and colleagues' chronic fatigue syndrome criteria and found that the Myalgic Encephalomyelitis International Consensus Criteria identified a subset of patients with greater functional impairment and physical, mental, and cognitive problems than the larger group who met Fukuda and colleagues' criteria.

The current study analyzed two discrete data sets and found that the Myalgic Encephalomyelitis International Consensus Criteria identified more impaired individuals with more severe symptomatology.

I do think this is academic to define ME and CFS separately. I do agree with the approach that Prof. Jason takes that a clearer symptom definition is required for decent research but the illness does not break down into CFS or ME.

Take depression as an example. There is a range of three factors influencing disease severity. These include low to high genetic involvement. Low to high metabolic-inflammatory involvement and low to high psychological involvement. If you are low on all three then your illness will be mild, you could be high on genetic but low on the other two, etc Or you could by high on all three and have very severe disease.

I suspect it is the same with ME/CFS:
High to low genetic involvement;
High to low metabolic-inflammatory involvement (including viral/bacterial infections, gut dysfunction, etc)
High to low psychological factors (including social and work factors)
The greatest difference appears to be between the metabolic-inflammatory and psychological dimensions. Some people will be at the psychological end of the continuum but they will not be devoid of the genetic and metabolic-inflammatory factors.

However for the purposes of research it is important to take people into different studies from each end of the continuum or at least to exclude those with high psychological factors in order to more clearly identify the biomarkers.